1. Field of the Invention
The present invention generally relates to a method and apparatus to detect the presence, absence, intensity and/or stability of a flame. More specifically, the present invention is directed to a novel method and apparatus which utilizes the modulating impedance within a given flame envelope generated as a result of the combustion process.
2. Description of the Prior Art
In a variety of industrial and other applications it is imperative to continuously evaluate and immediately identify the presence of, absence of, and/or the quality of a flame. Such applications include, for example, the burner flame within an enclosed vessel arrangement. If combustible gases are continued to be fed to such a vessel after the burner flame has been extinguished, a subsequent accidental spark may ignite these same gases thereby producing a catastrophic explosion. Similarly, in an open flare stack application it is also necessary to continuously monitor the flare pilots which ignite and burn vented gases. If the flare pilot is somehow extinguished during operation, noxious and/or combustible gases emanating from the stack, often heavier than air, will collect around the base of the stack thereby creating a risk and jeopardy to human life in conjunction with the attendant risk of fire or explosion.
A third example is seen in jet engines, and especially high performance military variants of jet engines, where "flare outs" sometimes occur thereby depriving the plane of significant operator control. In such instances, it is imperative that the absence of an ignitor flame be identified at an early stage so that remedial measures may be taken.
A number of devices have been developed to identify the presence or absence of a flame in response to the above and other situations. One of the more common of such devices is a flame detector often used in the ignitor of a flare stack which evaluates the direct current resistance of a flame as measured between two conductive, electrically isolated probes. This method of flame detection, commonly known as flame ionization detection, depends heavily on adequate electrical isolators between the probes for an accurate reading. In such devices, the presence of a selected conductivity to a direct current, usually in the range of 20-40 megohms, between the two probes is interpreted as indicating the presence of a flame.
Disadvantages with such systems reside in false or "ghost" signals created by contaminants and moisture which are often present in the burner chamber. When the electrical isolators which separate the probes become contaminated by moisture and dirt, their outer surface becomes conductive, thereby compromising the reading and rendering a false flame indication. The difficulty with flame ionization detection systems are thus compounded since the false signal is generated when the system remains in an "on" mode. This is colloquially referred to as a "non fail safe" condition.
A variety of other solutions to the difficulties and problems of flame detection have been proposed in the art. One such proposal is that for a remote sensor such as that disclosed in U.S. Pat. No. 3,586,468 as issued to Sims which discloses the use of an electromagnetic antenna provided in the vicinity of the flame. Flames are known to naturally generate electromagnetic waves which, according to Sims, are picked up by the antenna. Sims provides an ultrasonic signal to the burner to artificially produce variations in the flame at a characteristic frequency to aid in flame detection.
Other proposed solutions include the evaluation of different compression--rarefaction wave frequencies as disclosed in U.S. Pat. No. 3,233,650 as issued to Cleall or the evaluation of the acoustics of the burner chamber as disclosed in U.S. Pat. No. 2,767,783 as issued to Rowel et al. Disadvantages of these and similar techniques include, in the case of an acoustic detector, the difficulty of identifying a universally accurate and useful relationship between the acoustics of a given burner and the sound intensity created by the combustion of a given fuel/air mixture. Accordingly, the use of an acoustic apparatus often presents difficulty in retrofitting a burner already in operation.